Accelerometers are well known in the art. Typically, an accelerometer includes a return path (or top portion) and an end cap (or bottom portion). A reed, or proofmass, preferably constructed of quartz and supported by an integral frame, is placed between and supported by the return path and the end cap for pivotal movement (to a limited extent) in response to acceleration forces. Known sensing means measure these forces acting on the proofmass which must be allowed to pivot freely relative to its frame and the return path and end cap.
A preload band, in conjunction with an adhesive, binds the return path and the end cap together. The band is applied after a predetermined compressive force is imparted upon the return path and the end cap with the proofmass frame held therebetween. Applied in this manner, the preload band endeavors to maintain the imparted compressive load upon the frame of the quartz proofmass (actually upon the mounting pads of the frame) which lies between the return path and the end cap.
For the sensor means, and thus the accelerometer, to function properly, there must be a constant compressive load on the frame supporting the quartz reed. Accelerometer performance will be directly affected by any variations in the compressive load on the frame if these variations are imparted to the proofmass or reed itself.
Thus, from the outset, an accelerometer must have a constant and clearly defined load on the quartz reed frame. Unfortunately, such a preload is difficult to realize. A typical preload of 25 pounds requires only 0.00004 inches compression of the return path, end cap, and reed. Because accurate preloads are a necessity and achieving these accurate loads entails working with such minuscule lengths, it is very difficult to realize a high precision accelerometer.
Another problem with accelerometers arises after the preload is imparted. As a result of the various coefficients of expansion for the distinct components of the accelerometer, under varying temperature conditions, these components experience differential expansion. Variations in the preload result under these circumstances. Since coefficients of expansion are an inherent quality of the materials used, it is very difficult to combat this intrinsic deficiency in accelerometers.
A similar problem results when the adhesive binding the end cap and the return path begins to degenerate. Naturally, any adhesive will eventually lose its ability to bind. Consequently, there is an inevitable breakdown of the preload balance. Again, this degradation in performance is inherent in accelerometer design.